The present invention relates to thermoplastically processable polyurethane elastomers (TPU) with self-extinguishing properties, a process for their preparation and their use.
Thermoplastic polyurethanes (TPU) are of great industrial importance because of their good elastomer properties and thermoplastic processability. An overview of the preparation, properties and uses of TPUs is given e.g. in Kunststoff Handbuch [G. Becker, D. Braun], volume 7 “Polyurethane”, Munich, Vienna, Carl Hanser Verlag, 1983.
TPUs are usually built up from linear polyols (macro-diols), such as polyester-, polyether- or polycarbonate-diols, organic diisocyanates and short-chain, usually difunctional alcohols (i.e. chain lengtheners). They can be prepared continuously or discontinuously. The best known preparation processes are the belt process (see, for example, GB-A 1 057 018) and the extruder process (see, for example, DE-A 19 64 834).
Thermoplastically processable polyurethane elastomers can be built up either stepwise (e.g. by the prepolymer metering process) or by simultaneous reaction of all the components in one stage (e.g. by the one-shot metering process).
To achieve a good flame resistance in polymers, various types of additives can be employed. Halogen-containing materials, inorganic fillers or organic phosphorus compounds or mixtures thereof are often employed for this. An overview of the use of flameproofing agents in polymers is given e.g. in H. Zweifel, Plastics Additives Handbook, 5th ed., Hanser Verlag Munich, 2001, chapter 12; J. Green, J. of Fire Sciences, 1997, 15, p. 52–67 or Kirk-Othmer Encyclopedia of Chemical Technology, 4th ed., vol. 10, John Wiley & Sons, New York, p. 930–998.
Various substances or mixtures thereof are possible as flameproofing agents in thermoplastically processable polyurethane elastomers (TPUs).
European Patent (B) 0 508 072 describes the use of melamine in amounts of 30 to 40 wt. % as a flameproofing agent in TPU. The TPU treated in this way has a tensile strength of 15 MPa and is rated with UL-94 V-0.
The application WO 00/66658 and EP-B 617 079 describe combinations of phosphates, phosphonates and/or derivatives thereof in combination with melamine cyanurate. The corresponding TPUs have tensile strengths of ≦35 MPa and the flameproofing is rated with UL-94 V-0.
The sharply decreasing level of mechanical properties, caused by the necessary high amounts used, has proven to be a problem in the use of inorganic-based flameproofing agents or of melamine derivatives. An undesirable migration of the organic derivatives employed, e.g. the phosphates or phosphonates, is also observed.
The object of the present invention was therefore to provide thermoplastically processable polyurethane elastomers (TPUs) which are self-extinguishing, and simultaneously exhibit a low shrinkage and a high tensile strength, and show no or only slight blooming effects.
It has been possible to achieve this object by building into the TPU, via the so-called prepolymer process, certain phosphorus-containing compounds which can be built in.
Diol-phosphonates as flameproofing agents which can be built in are known in the field of polyurethane foams and described in DE-C 25 20 180. With these flameproofing agents thermoplastic polyurethanes are obtained which have too high shrinkages.